For some Florida voters, last weekend started on a low note — literally.

Last Friday, letters that questioned registered voters’ eligibility began appearing in mailboxes across the state, NBC News reported. The letters, which resemble an authentic message from the Florida Division of Elections with a letterhead and contact information, notify recipients that the department has gleaned “information” concerning “citizenship status, bringing into question your eligibility as a registered voter.” The document also informs voters that they must complete a Voter Eligibility Form and return it to the Supervisor of Elections Office within 15 days or else they will surrender their eligibility to vote. Chris Cate, a spokesperson for the Florida Secretary of State’s Office, told NBC that as of Oct. 24, state officials have received reports of between 50 and 100 of these letters from individuals in at least 28 counties. Cate also told NPR that although voters in both parties have received the mailings, most of which were sent from Seattle, Republicans constitute the bulk of the addressees.

Officials were able to identify the fraudulent letters because of the waiting period’s inconsistency with the genuine alerts, which allow residents 30 days to send a completed Voter Eligibility Form. The legitimate messages are also sent by certified mail instead of standard mail, the method of choice for the fakes, the Fort Myers, Fla. News-Press reported.

The phony notes dampened the weekends of some flagged voters, including Jeff Siewert, a resident of Naples — located in Collier County — who received one last Friday.

“I was born in the U.S.,” Siewert told the News-Press. “I’m 65 years old. I’ve voted in multiple elections since the mid-’60s, and I was a little taken aback.”

Siewert spent the weekend worried that his absentee ballot would be voided, but county officials appeased his fears on Monday when the election office opened.

“I was just concerned that for one reason or another my vote wouldn’t be counted,” he said. “And in this election, every vote counts.”

The FBI opened an investigation for voter intimidation on Wednesday following the stream of reports about the fake mailings, NBC reported. If caught, the individual(s) responsible could be fined up to $5,000 and spend up to five years in jail, according to an email sent by Florida’s Secretary of State Ken Detzner to election supervisors.

“We’re taking it as a serious situation,” said David Couvertier, spokesperson for Tampa’s FBI office. “We’re looking at everything from civil rights violations to election fraud — to everything in between.”

The false letters aren’t the first reported attempt in this year’s election to deceive voters. On Oct. 16, Florida’s election protection hotline warned of misleading phone calls claiming that a new law permits voters to cast their ballots via phone by simply providing their names and addresses. The Virginia State Board of Elections also warned voters of similar calls in the state, which targeted older residents.

Cate advised voters to remain alert about possible fraud.

“This is an example of why voters need to be vigilant during the election season and to be aware that there may be people out there committing fraudulent acts,” he told NPR.